Thermal Substrate

This patent application claims priority from provisional U.S. patent application No. 63/636,658, filed Apr. 19, 2024, entitled, “THERMAL SUBSTRATE,” and naming John P. Ciraldo as inventor, the disclosure of which is incorporated herein, in its entirety, by reference.

Illustrative embodiments generally relate to thermal management in semiconductor devices and, more particularly, illustrative embodiments relate to integrated circuits that incorporate diamond-based substrates,.

Thermal management of integrated circuits (ICs) is significant for several reasons, primarily related to the physical properties of the materials involved and the operational reliability and efficiency of the devices. When ICs operate, they consume electrical power, a portion of which is converted into heat due to the resistance in the materials and the switching activities of transistors. This is particularly pronounced in high-performance devices like CPUs, GPUs, and high-speed memory, where billions of transistors switch on and off billions of times per second. Excessive heat can lead to thermal stress on the materials in the IC, potentially causing physical damage or degradation over time. Materials expand when heated and contract when cooled; repeated thermal cycling can cause fatigue in the materials, leading to cracks and other failures. High temperatures can also accelerate electromigration, a process that gradually degrades the pathways in the chip. Many semiconductor materials, including silicon, have properties that vary with temperature.

In accordance with an embodiment, a diamond thermal structure includes a diamond heat spreader layer that has been enriched in carbon-12 isotope, and a diamond thermal sink layer positioned beneath the heat spreader. The thermal sink layer contains diamond with a non-enriched isotopic composition.

In various embodiments, the carbon-12 atomic concentration in the heat spreader may equal or exceed 99.5%. The thickness of the heat spreader layer may range from 1 micron to 20 microns. The thermal sink layer may have a carbon isotope ratio substantially matching that of methane found in nature. The heat spreader and thermal sink may be deposited in a continuous chemical vapor deposition process, switching gases without stopping deposition. Alternatively, the structure may be formed by growing the heat spreader on a polished substrate, then growing the thermal sink atop the spreader, followed by separation from the substrate to expose the heat spreader at the top.

In some embodiments, the composite structure also includes a wide bandgap or ultra-wide bandgap semiconductor device thermally coupled to the top surface of the diamond heat spreader.

In accordance with another embodiment, A method of fabricating a diamond thermal structure deposits a diamond heat spreader layer on a substrate using a carbon-containing precursor gas enriched in carbon-12. The method continues by switching to a carbon-containing gas with an unenriched isotopic mix, and then depositing a thermal sink layer atop the heat spreader. After deposition, the diamond structure is separated from the substrate to expose the heat spreader.

The enriched precursor gas may have a carbon-12 concentration equal to or greater than 99.5%. The heat spreader layer may be grown to a thickness between 1 and 20 microns. The entire process can be performed in a continuous CVD system without breaking vacuum. The substrate may be made of single-crystal or polycrystalline diamond, and separation may be achieved using laser cleaving, ion implantation, or a sacrificial release layer. The separated structure may be flipped to position the heat spreader at the top surface, which may optionally be polished to prepare for bonding to a semiconductor device.

A method of fabricating the diamond thermal structure begins with depositing a thermal sink layer on a substrate using a non-enriched carbon-containing gas. The surface of the thermal sink is then polished, and a heat spreader layer is deposited using a precursor gas enriched in carbon-12.

In this forward process, the enriched precursor gas may include more than 99% carbon-12. Preferably, greater than 99.5% carbon-12. The heat spreader may be between 1 and 20 microns thick. Both the thermal sink and spreader layers can be deposited via chemical vapor deposition. The polishing process may achieve a surface roughness of less than 10 nanometers RMS. Suitable substrates may include diamond, silicon, sapphire, or silicon carbide. After depositing the heat spreader, its surface may also be polished to enable bonding. The layers may be bonded directly without an intermediate adhesion layer.

An integrated circuit includes a wide bandgap or ultra-wide bandgap semiconductor device that generates heat. This device is coupled to a C12-enriched diamond heat spreader, which in turn is connected to a diamond heat sink having a natural isotopic composition of C12 and C13.

In some cases, the heat spreader is thermally connected at the heat-generating portion of the device. The heat spreader and heat sink may be either grown together or bonded. The heat spreader may have a thickness that is% or less of the heat sink layer. The semiconductor device may be a transistor. In addition, the structure may include a larger metal heat sink thermally connected to the diamond heat sink. This secondary heat sink may include fins and be made from copper or other conductive metals.

In accordance with yet another embodiment, a method dissipates heat from an electronic device by growing a diamond substrate in a CVD chamber. The process first grows a diamond heat sink layer using a carbon source with one concentration of C12, followed by growth of a heat spreader layer using a source with a higher C12 concentration. The spreader is then thermally coupled to an electronic device.

This method may include attaching the diamond heat sink to a non-diamond heat sink, such as a copper structure. Heat is generated in the device at a specific region, and transferred into the heat spreader and then the sink. The heat sink may be 50 to 1200 microns thick, more typically 100 to 550 microns. The heat spreader may be 50 nm to 50 microns thick, more typically 1 to 50 microns, and may have a thickness less than 25-33% of the total diamond structure.

The method may be applied to transistors or other devices. Another embodiment includes providing a diamond structure with two surfaces, where the surface touching the electronic device contains a higher concentration of C12 than the opposite side. The device may again be a transistor, and may be paired with a large external heat sink.

Additional embodiments may involve multiple devices or doped junctions, and may be used in high-frequency and high-power systems. Gate sizes of the devices may range from 100 nm to 5 microns. Devices using the disclosed structure may achieve energy densities between 2 W/mm and 20 W/mm. The heat spreader layer may have C12 isotopic purity of at least 99.9%, as measured by mass spectrometry.

Illustrative embodiments provide a thermal substrate for integrated circuits, featuring a dual-layer diamond structure that enhances heat management in high-power and high-frequency semiconductor devices. The system integrates an ultra-high thermal conductivity heat spreader composed of isotopically pure Carbon-12 (C12) diamond directly beneath the heat-generating regions of the chip, such as GaN transistor gates, to laterally distribute heat and mitigate thermal hotspots. Beneath this, a more economical diamond heat sink layer—having isotopically standard or isotopically unpure diamond (C12/C13)—provides bulk thermal dissipation. Grown monolithically or bonded together using chemical vapor deposition (CVD), the layered diamond substrate enables improved device cooling, increased energy density, and reduced footprint, with applications in RF electronics, power conversion, and aerospace systems.

In illustrative embodiments, diamond and gallium nitride (GaN) are combined in high-power electronic and optoelectronic devices primarily to enhance thermal management. GaN has high electron mobility and ability to operate at high temperatures and voltages, making it suitable for, among other things, high-performance transistors, LEDs, and laser diodes. However, GaN devices generate significant amounts of heat, which can affect performance and reliability. Diamond has an exceptionally high thermal conductivity, which makes it an ideal material to draw heat away from the GaN components. By integrating a diamond layer, either as a substrate or a heat spreader, the heat generated by the GaN can be efficiently dissipated. This helps in maintaining the operational temperature of the GaN device within safe limits, thereby enhancing performance and extending the lifespan of the device.

Growing diamond on GaN can be challenging due to lattice mismatch and different thermal expansion coefficients between diamond and GaN. Thus, various embodiments may grow buffer layers between the diamond and the GaN to help manage the lattice mismatch and ensure a better mechanical and thermal connection. Materials like aluminum nitride (AlN) may be used as buffer layers. However, the use of the buffer lay undesirably reduces the ability of the diamond to conduct heat from the GaN.

Some other embodiments may bond diamond heat spreaders to GaN devices. This is simpler than integrating diamond as a substrate and can be retrofitted to existing designs. GaN devices are grown on a foreign substrate, the substrate may be thinned out and bonded to the diamond for heat conduction.

Some embodiments may use thin diamond films deposited onto GaN devices, though this method might not provide as efficient thermal conductivity as thicker, bulk diamond.

In illustrative embodiments, diamond is advantageously made highly thermally conductive by controlling the isotope of the diamond. The highly thermally conductive diamond functions as a heat spreader. The heat spreader conducts heat away from the chip (e.g., the GaN) itself. Since C12 diamond is highly thermal conductivity, it is effective at quickly absorbing and spreading out the heat. By spreading the heat across a larger area, the C12 heat spreader helps in more effective heat dissipation. The heat spreader acts as the interface between the chip and any additional cooling mechanisms, such as heatsinks or liquid cooling systems. It ensures that heat is evenly transferred to these cooling solutions. In various embodiments the diamond also protects the delicate GaN of the chip from physical damage and from direct contact with the cooling solutions, which might otherwise cause stress or damage due to uneven surfaces or pressure. Illustrative embodiments are discussed below.

Carbon has two stable isotopes and several radioactive isotopes. The two stable isotopes are Carbon-12 (C12) and Carbon-13 (C13). Natural diamonds are primarily composed of the carbon-12 (C12) isotope, which is the most abundant isotope of carbon, constituting about 98.9% of all carbon in nature. Because of this predominance, carbon-12 is also the major component of all naturally occurring organic compounds, including diamond.

Carbon-13 (C13) is also present in diamonds, but in much smaller amounts, reflecting its overall natural abundance of about 1.1%. The specific ratio of carbon-12 to carbon-13 in a diamond can vary slightly based on the diamond's origin and the carbon sources from which it was formed. This ratio is sometimes used by geologists to trace the processes and origins of diamonds, as different types of organic materials and geological processes can leave distinct carbon isotopic signatures.

schematically shows an integrated circuit (IC), also known as a microchip or just a chip in accordance with illustrative embodiments. The IC is a tiny electronic device made out of a small piece of semiconductor material, usually silicon, GaN, etc. On this small chip, thousands, millions, or even billions of electrical devices like transistors, diodes, resistors, and capacitors are fabricated closely together. These devices are interconnected to perform various electronic functions such as amplifying signals, computing, or storing data.

For simplicity,shows a single device, for example, a transistor on a substratein accordance with illustrative embodiments. The transistor may have a source contact, a drain contact, and a gate. Most of the heatin a transistor is generated in the regionbetween the sourceand the drain,known as the gate. The area where heat is generated may be referred to as a heat generating areaor a heat generating portion. Other devices also have heat generating areas. For example, in a diode, most of the heatis generated at the junction where the p-type and n-type semiconductor materials meet. In a resistor, heatis primarily generated throughout the body of the resistor itself. Those skilled in the art can determine heat generating areas of various devices.

In illustrative embodiments, the one or more devicesare on a diamond substrate. Although the substratemay be relatively large, a heat spreader portion(also referred to as the heat spreader) of the substrateis adjacent to the heat generating areaof the device. Various embodiments provide a C12 heat spreading portionunderneath the heat generating portion. Then, the remainder of the diamond substratemay be a combination of C12 and C13 that operates as a bulk heat sink portion(also referred to as the heat sinkor substrate-level heat sink). Thus, heatfrom the relatively small heat generating areais spread across a larger heat spreading portion, which pulls the heat towards a larger heat sink portion.

C12 isotopically pure diamond can be expensive to manufacture. Accordingly, illustrative embodiments provide C12 diamond in the heat spreading portion.

To create isotopically pure diamond, isotopically pure methane gas may be used. Illustrative embodiments may thus grow diamond in a CVD chamber using regular methane gas. At some point during the growth process, when it is desirable to grow isotopically pure C12 diamond, an isotopically pure methane gas may be used. In various embodiments, the heat sinkmay be grown to hundreds of microns of thickness, and typically 100-550 microns. In various embodiments, the heat spreading filmmay be 1-50 microns on top of the heat sink portion, although the final thickness is dependent on the features applied. The filmsdo not be require additional treatments, such as is the case in quantum applications, due to them being used only as a passive heat spreader. Various embodiments grow the diamond heat spreaderusing isotopically pure methane gas. Optionally, pressure, nitrogen concentration, and growth conditions may otherwise remain substantially the same when growing the heat spreaderand when growing the heat sink.

schematically shows a detailed side view of the heat generating portionof the devicerelative to the heat spreading portionin accordance with illustrative embodiments. In various the heat spreading portionhas a thickness that is proportional to the heat generating portion(e.g., the gatesize). This allows a thin film of C12 at the top surface that has excellent heat extraction throughout the substrate. The substratethus advantageously acts very similar to a substrateformed entirely of C12 without the expense of fabricating the entire substrateof C12.

In some embodiments, R=thickness of the heat generating portionof the device (Td) (e.g., the thickness of the gate). In various embodiments, the heat spreading portionmay have a thickness (Tsr) of between about R and about R{circumflex over ( )}2. In general, the thickness of the heat sink portion(Tsk) is greater than the thickness of the heat spreading portion(Tsr). In various embodiments, the heat sink portionmay be between about 100-550 microns thick. Some embodiments may be as thin as 50 microns.

schematically shows a top view ofin accordance with illustrative embodiments. The heat spreader, heat sink, and heat generating portionmay all be considered to have an area defined by a length multiplied by a width. This is not intended to imply that the area must be rectangular, but merely is shown for discussion purposes. In some other embodiments, the area may include a radius, or may be defined by other dimensions (e.g., of a non-regular shape). Any of the aforementioned areas may have a rectangular or non-rectangular shape.

The area of the heat spreaderis generally the same as or larger than the area of the heat-generating portionit is designed to cool. In various embodiments, the heat generating portioncan be said to have an area roughly defined by a lengthand a width. When there are multiple deviceshaving multiple heat generating portions, the area of the heat spreading portionis generally larger than the sum of the area of the heat generating portions. The area of the heat sinkis generally the same as or larger than the area of the heat spreader. The heat spreaderhas an area (particularly for a top surface that interfaces with the heat generating portion(s)) that is defined by a lengthand width. In a similar manner, the heat sinkhas an area (particularly for a top surface that interfaces with the heat spreader) that is defined by a lengthand a width.

Thus, various embodiments advantageously provide a C12 pure heat spreaderthat distributes heat across its surface. The heat spreaderhas a given isotopic purity (i.e., the proportion of a particular isotope (in this case, Carbon-12) relative to all other carbon isotopes present in the material). As described herein, the heat spreadercan be considered C12 pure if it is equal to or greater than 99.5% isotopic purity. In various embodiments, the heat spreaderhas greater than 99.8%, or greater than 99.9% C12 isotopic purity. Indeed, various embodiments achieve greater than 99.95% C12 isotopic purity for the heat spreader. Various embodiments could, in theory, form the heat spreaderfrom isotopically 100% pure C12 diamond to achieve optimal thermal conductivity. However, due to current practical manufacturing limitations, such levels of purity are not yet widely achievable at scale. Illustrative embodiments are not intended to be limited by current manufacturability, and in some embodiments, upper isotopic purities may range up to approximately 99.998% C12, depending on available deposition and purification technologies.

This ratio can be measured using mass spectrometry techniques, including ToF SIMS. This helps in managing hot spots on the chip that generate more heat, thus preventing any single point from becoming excessively hot. In various embodiments, the heat spreaderis advantageously formed from a higher concentration of C12 diamond (than the remainder of the diamond) that has excellent high thermal conductivity, to facilitate quick heat distribution. The heat spreaderis directly in contact with the heat-generating component, like the heat generating portion, and acts as the first level of heat distribution. The heat spreaderspreads out the heat generated by the chip to a larger area, making it easier for the subsequent cooling mechanisms (like the heat sink) to dissipate this heat effectively.

Furthermore, illustrative embodiments provide a monolithically grown diamond heat sinkwith the heat spreader. The heat sinkdissipates heat into the surrounding environment, typically the air inside the casing of the device or directly to a cooling fluid in more advanced systems. The heat sinkis formed of a lower concentration of C12 diamond than the heat spreader(e.g., diamond that may include C12 and C13, and is advantageously more economical), which increase the surface area that interacts with the air or cooling medium. The heat sinkis advantageously grown on the heat spreader(or vice versa) using, for example, CVD. In some other embodiments, the heat sinkmay be otherwise attached to or placed on top of the heat spreader. In some embodiments, the heat sinkcan be part of a more complex cooling system that includes fans, heat pipes, or liquid cooling loops. By maximizing the surface area exposed to air (or another coolant), the diamond heat sinkenables efficient heat transfer from the electronic component to the environment, thus cooling the component. While the heat spreaderevenly distributes the heat from the chip, the heat sinkremoves this heat by allowing it to dissipate into the environment.

Illustrative embodiments advantageously allow for improved heat management, and therefore, for smaller devices. In various embodiments, the size of the transistor may be reduced relative to current state of the art transistors. The devices may be used for high-power high-frequency applications. This includes applications where there is a wide bandgap (e.g. GaN/SiC) or ultra-wide bandgap (ex: AlGaN, Gallium oxide, AlN) device positioned above the heat spreader. HEMT devices and MMICs are examples, but any device operating near or above 1 kV, or at operational frequencies above several hundred GHz may be applicable. For example, bandgaps may be above 2 eV and up to, for example, 7 eV. In various embodiments the devices may have an energy density of greater than 2 W/mm. With improved thermal conductivity, the inventors believe it is possible to achieve operation of devices having significantly increased energy density of up to 20 W/mm or even 40 W/mm.

schematically shows a top view of the heat generating portionhaving multiple semiconductor devicesin accordance with illustrative embodiments. Certain figures may illustrate only a single semiconductor device, such as a transistor, for the purpose of simplifying the drawing and clearly explaining the interaction between components. It should be understood, however, that in practical implementations, the semiconductor device layer may include millions or even billions of densely packed transistorsand other active components, depending on the application and chip architecture.shows a plurality of semiconductor devicesto better convey the concept of the heat-generating footprint, including how heat-producing elementsare distributed across a defined area. While the number of devicesillustrated is far fewer than would be present in a real-world implementation, the depicted arrangement is intended to help visualize the concept of dense device integration and the continuous thermal zone that is addressed by the heat spreader layer. The illustrative quantity and spacing of devicesin the figures should therefore be understood as schematic and non-limiting.

As shown in, in some embodiments, the top surfacearea of the carbon-12 enriched heat spreader layeris dimensioned to match or exceed the area over which heat is generated by the semiconductor device layer. In this example, the length of the heat generating areacan be said to have an area roughly defined by a lengthand a width. The area of the heat spreader, defined by the lengthand the width, may match or exceed the area of the heat-generating footprint

Because modern integrated circuits often include billions of densely packed transistors and other active components, these generate heat over a tightly packed region of the chip. For thermal management purposes, it is not always practical to calculate the precise combined surface area of the individual heat-generating elements. Instead, in many implementations, the heat-generating region is characterized by a total footprintor continuous perimeter that encloses the active device array. This area defined by the perimeterprovides a practical and functionally meaningful estimate of the thermal region to be covered by the C-12 layer.

shows a dashed-line box drawn around the semiconductor devicesto illustrate the approximate location of the heat-generating footprint. While the boxmay not precisely correspond to the exact locations of all individual transistors or active regions, it is intended to generally indicate the area over which heat is generated during device operation. This region is typically well understood during chip design and layout, and serves as a practical basis for sizing the carbon-12 heat spreader layer.

The heat-generating footprintrefers to the planar surface area on or within a semiconductor device structure over which thermal energy is produced during operation. This area typically encompasses the collective lateral extent of transistors, diodes, interconnects, and other active circuit elements that dissipate power as heat. In practice, the heat-generating footprintmay be defined by a continuous perimeter or bounding shape that encloses all such heat-generating components, including densely packed arrays of devices with minimal spacing. Because the spacing between individual devicesis often small relative to the device size (e.g., sub-micron), the entire enclosed region may be treated as a single continuous thermal zone for purposes of thermal design and heat spreading.

The heat-generating footprintmay also be characterized based on known design dimensions of the semiconductor die or chip, especially when detailed device-level mapping is impractical. This area serves as the reference region over which a heat spreader layer—such as a carbon-12 enriched diamond layer-should be positioned or sized in order to effectively collect and redistribute thermal energy.

The carbon-12 heat spreader layeris typically deposited or bonded as a continuous, uninterrupted layer. Due to the nature of CVD growth or wafer-scale bonding, it is difficult to confine deposition to isolated local regions or individual hotspots. Therefore, in most embodiments, the heat spreaderis a monolithic layer that underlies the entire active region of the device chip and extends beyond it in some cases.

To function effectively, the C-12 heat spreader layershould preferably have a surface areathat is at least equal to the area defined by the perimeterenclosing the heat-generating semiconductor structures. In some embodiments, the heat spreader layerhas a top surface areathat is no smaller than 70% of the heat-generating area, more preferably at least 85%, and most preferably greater than or equal to 95%, in order to ensure adequate lateral phonon transport and efficient heat redistribution.

In other embodiments, the C-12 layer may be larger than the defined thermal perimeter, in order to provide thermal margin or to simplify manufacturing. However, because isotopically enriched carbon-12 material is expensive, unnecessarily large C-12 layers may increase cost without proportional thermal benefit. Therefore, in some embodiments, the surface areaof the C-12 layer is no more than 150%, or no more than 125%, of the area defined by the heat-generating device perimeter. This maintains a practical trade-off between thermal effectiveness and material cost.

The C-12 heat spreader layermay also be dimensioned in reference to the continuous area of the overlying heat-generating semiconductor region, rather than a sum of individual device elements. Because of the extreme density of modern transistors and the minimal spacing between them, treating the device layer as a continuous thermal source simplifies the design and ensures that heat from all active regions is effectively intercepted by the spreader layer.

In one example, a high-power semiconductor device, such as a GaN-based RF amplifier or processor die, may have a heat-generating footprintdefined by a continuous region measuring 10 millimeters by 10 millimeters, resulting in a total active thermal area of 100 mm. This footprintcorresponds to the lateral dimensions of the semiconductor die or the portion of the die containing densely packed, heat-generating circuit elements such as transistors and interconnects.

To effectively spread heat laterally from this footprint, the carbon-12 enriched diamond heat spreader layeris preferably fabricated with a top surface areaequal to or greater than the heat-generating footprint. In some embodiments, the C-12 layer has a surface area that closely matches the device area, such as 10 mm×10 mm (100 mm). In other embodiments, the spreadermay extend slightly beyond the perimeterof the active region—for example, 11 mm×11 mm (121 mm)—to provide margin for alignment tolerance or enhanced lateral thermal conduction.

To avoid unnecessary cost due to the use of isotopically enriched material, the C-12 layeris preferably not substantially larger than required. For example, in certain cost-sensitive embodiments, the surface areaof the C-12 layer may be constrained to no more than 125% of the heat-generating footprint(e.g., 125 mmin this example), and more preferably no more than 110-115%, to balance thermal performance and material efficiency.